A variety of applications use polyphase direct current (dc) spindle motors to generate rotational motion. In particular, applications such as hard disk drives and CD-ROM type drives often use polyphase dc spindle motors, such as three-phase dc motors, to rotate information platters such as the magnetic disks of a hard disk drive. Three-phase dc motors are brushless motors that generate rotational motion by selectively or commutatively providing six currents through sets or pairs of coils in a stator winding using a commutation control circuitry.
The speed of the rotational motion is controlled through the six currents applied to the stator winding coils. For example, the stator windings of a three-phase dc motor may be coupled in a "Y" configuration and include an A-coil, a B-coil, and a C-coil, all coupled at one end at a center tap node. The remaining ends of each coil are selectively or commutatively coupled to either a high side driver, a low side driver, or to an open circuit, as controlled by the commutation control circuitry.
During steady state operation, current flows from a high side driver, through a first coil coupled to the high side driver, through the center tap, through a second coil coupled to the low side driver, and to the low side driver. During this time, a third coil couples to the center tap on one end while the other end is provided as an open circuit. After a period of time, a commutation occurs so that current may now flow through the third coil and either the first coil or the second coil. A commutation is the transfer of current from one path in a circuit to another. Thus, current flows through the first coil and the second coil during a steady state operation until a commutation occurs, at which time, current then flows through either the first or the second coil and the third coil until the next commutation occurs.
A total of six currents may be provided in the stator windings of a three-phase dc motor through six commutations. The current flows, for example, may be provided through the stator coils in the following sequence: A-coil to C-coil, A-coil to B-coil, C-coil to B-coil, C-coil to A-coil, B-coil to A-coil, and B-coil to C-coil.
Each stator winding coil includes an associated resistance and inductance. As the current through a stator winding coil is switched off, as a result of a commutation or because the power is shut off, the inductance of the stator winding coil generates a corresponding inductive voltage spike according to the equation: V=L(di/dt). This voltage spike may also be referred to as a flyback voltage or inductive "kick," and is generated in response to the changing current in the coil. The inductive voltage spike, if uncontrolled, often will exceed the breakdown voltage of the associated control circuitry, such as the commutation control circuitry, resulting in its destruction.
The inductive voltage spike may be controlled using either an external or an internal Schottky power diode to provide a current path to dissipate the energy stored in the coil. These diodes may provide a current path to dissipate, for example, up to two amps. The Schottky power diodes are provided for each coil of the stator winding and may be discrete, external Schottky power diodes or internal Schottky power diodes fabricated as part of an integrated circuit. Problems arise when using Schottky power diodes to control the inductive voltage spike. External Schottky power diodes are expensive, about twenty-five cents a piece, bulky, and add significantly to overall circuitry cost. Internal Schottky power diodes take considerable die or circuit area resulting in significantly larger overall integrated circuitry.
Other solutions to the flyback voltage problem include the use of active devices, such as comparators, to continually monitor a voltage to determine if it exceeds a particular reference voltage. If the monitored voltage exceeds the reference voltage, other circuitry may be implemented to dissipate or reduce the voltage. However, these solutions suffer a serious drawback. The use of active devices, such as the comparators, require a power supply voltage, and thus, the circuitry will not operate when power is shut off during power down or in the event of a power loss. When power is shut off or lost, the currents through the stator winding coils are changing and, hence, are generating flyback or inductive voltages at high levels. These flyback voltages often exceed the breakdown voltage of the control circuitry resulting in its destruction.